Downhole fluid analysis (DFA) is a rapidly growing discipline in wireline logging and has become a keystone in reservoir evaluation. DFA addresses the failed and overly optimistic assumption that oil reservoirs consist of “one giant tank of homogeneous hydrocarbon.” DFA can be used to find compositional gradients as well as to identify compartments. DFA also can be used to establish reservoir connectivity. DFA is typically based on bulk optical spectroscopy, which is useful to determine concentrations or ratios of components in sampled fluid.
Schlumberger has pioneered the use of optical and other spectroscopic techniques to analyze formation fluids downhole. For example, Schlumberger has introduced an optical measurement technique to estimate gas-oil ratio (GOR) of formation fluids, and determine hydrocarbon and gas composition for C1, C2-C5+, and C6+. While these advances have been impressive, it is still common practice to send samples of the formation fluids and transport them to the laboratory for further detailed analysis. Current downhole analysis techniques do not provide quantitative measurement of the individual hydrocarbon moieties for C2, C3, C4 and C5 and molecules with more that six carbons are undistinguishable.
Another challenge with DFA is the time used to extract a useful sample of reservoir fluid. Using a conventional fluid sampling probe a long time can be taken up pumping fluid until it is suitably free of filtrate contaminants. Using a focused probe, such as one with two flowlines speeds up the time needed for extractions. However, it is also desirable to speed up the process while not sacrificing spatial resolution.